1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive employing method of writing a block of data to a data region following detection of a defective servo sector.
2. Description of the Prior Art
Hard disk drives store large volumes of data on one or more disks mounted on a spindle assembly. Disk drives employ a disk control system for interfacing with a host (e.g., a computer) to control the reading and writing of data on a disk. Each disk includes at least one disk surface which is capable of storing data. On each disk surface, user data is stored in concentric circular tracks between an outside diameter and an inside diameter of the disk. Servo systems are employed to maintain alignment of a transducer head with a desired target data track (termed "track following"), for reading and writing user data on the disk surface within desired control parameters.
Embedded servo systems store servo data on the same disk surface as user data to provide control signals and information employed in the operation of the servo system. User data on the disk surface is divided into groups of data sectors. Embedded servo information is recorded in servo sectors placed in radially continuous narrow wedges between the groups of data sectors. In this regard, servo sectors are commonly referred to as "servo wedges". For example, a concentric data track may include 60 equally spaced servo sectors with data regions (i.e., a region containing data sectors, groups of data sectors or partial data sectors) located between adjacent servo sectors.
Each servo sector includes track identification and tracking information used in track seeking and following operations. For example, the track identification information may include track number or address and sector number, and the tracking information may include a field for establishing automatic gain control (AGC) and phase lock oscillator information (PLO) settings, timing information (e.g. a servo sync word) and servo burst information for positioning a transducer head over the disk surface.
During execution of a command to read or write data to a target data sector on the disk surface, servo information is sampled each time a servo sector passes under the associated transducer head. Sector timing is maintained by detecting a servo sync word field for each servo sector within a timing window, wherein the timing window is set based on detection of the previous servo sync word field within a previous servo sector.
If the servo system does not detect a servo sync word within the timing window for the servo sector preceding the target data sector, a servo sync word timeout signal is generated and the servo processor may indicate that writing is unsafe (i.e., inhibits write gate) due to a defective servo sector or motor speed variation. With write gate disabled, user data is not written to the disk surface.
The servo sector may be defective due to a transient error or a "hard" sector error. A transient error is defined as an error or defect which clears over a period of time. For example, a transient error may occur due to a thermal asperity on the disk surface. A retry mode may be entered, wherein the command is retried a number of times allowing sufficient time to pass for the transient error to clear. A hard sector error conversely is not transient in nature. Hard sector errors include defects in the disk surface, which may be a result of the manufacturing process or handling of the disk drive. Hard sector errors are more permanent in nature, and do not clear using the retry process, therefore the sector in question is a defective servo sector.
The existence of a defective servo sector may become known during manufacturing testing, or during customer use. If during manufacturing testing or during later operation of the disk drive a servo sector is found to be defective due to a hard sector error, the disk drive controller conventionally will not allow read or write commands to be executed which are directed to the data sectors located between the defective servo sector and a next consecutive servo sector. A defect management scheme may be utilized to reallocate the data sectors following the defective servo sector to a sparing region. To facilitate recovery from a sector error, data is allowed to be read from the data sectors following the defective servo sector. Because of the more critical nature of writing data on the disk surface, the occurrence of a defective sector during normal use conventionally requires re-allocation of data sectors following the defective servo sector to a sparing region elsewhere on the disk. In the case of a defective servo sector, the following data region may in fact be perfectly capable of storing data reliably, therefore re-allocation of the data sectors consumes a severely limited commodity in the disk drive, i.e. the sparing region.
There is a need for a more efficient way to handle the impact of a defective servo sector.